Alternate Universe/Headscratchers

How do I know if there isn't another copy of me with a totally different life in another universe? Or one where my twin brother was alive? I guess it's like a Choose Your Own Adventure book of my life then.
It's sort of....demotivational to think that there is an infinite number of copies of me who have made every possible choice I could have taken.
That's the point of Larry Niven's short story "All the Myriad Ways". A detective investigating a rash of unaccountable suicides works out that the vic/perps have been de-motivated by evidence proving that the Garden of Forking Paths interpretation of QM is true.
Alternate universes have been proven to exist? Whoot! When was that?
- Proof, or this theory didn't happen!
  - It's kind of the corollary to String Theory. Namely, we might live on a 3+1 plane, ergo there may be other 3+1 planes out there.
- String theory has been proven to be accurate? Whoot! When was that?
  - You're missing the point it's just the idea that bothers him. Tropes are basically just ideas anyway.
    - I guess if you look at it from that perspective, it can seem very depressing. But look on the bright side: At least you aren't in a universe comprised only of shrimp!
- There's also many worlds. While it's possible that the correct explanation is something nobody thought of, the only current competing theory (Copenhagen interpretation) has alternate universes on a smaller scale. For example, if you send a photon through two slits, there's one universe where it goes through the left slit, and one through the right slit. According to the Copenhagen interpretation, said universes are composed only of that photon, and exist entirely within "the universe". The problem here is that means that it has all the laws many worlds has, along with additional laws for particles becoming entangled, becoming unentangled, and interacting while not entangled. As a result, it's vastly more complicated, and there's no evidence for it. Probability decreases exponentially as complexity increases, so we can safely say it's false.
  - That is Many Worlds Theory, it is not Copenhagen Interpretation. Bohr declined to speculate about the state of the photon before it hit the measuring machine. Physics is all about measuring stuff; speculating about the state of stuff which is not being measured ain't physics. If you shoot photons down a wave measuring machine, it will tell you what kind of waves they are, a particle measuring machine will tell you what kind of particles they are and a jam measuring machine will tell you what flavour of jam they are. That is what measuring machines are there for.
If it makes you feel any better tell yourself that most of them lived in Universes that the Large Hadron Collider (or Black Hole Machine) destroyed. That is the reason it kept failing and breaking down in this one, we are in the reality where a breakdown spares us an eternity of suffering at the tentacles of an Eldritch Abomination that results from it actually finding the Highs Boson.
The existence of a single possible future isn't any less depressing.
Depends on your perspective, I suppose; the inverse of this logic is that they will never be able to make the choice you end up making.

I'm bugged by the trope In Spite of a Nail when applied to Alternate Universe or time travel. Alternate words that have a large divergence (Nazis Won Earth, etc. ), but alternate versions of the main character still exist. If the world is that different I'm doubting your parents still met under the same circumstances, if at all.

Well,there are a near-infinite number of universes.So while an alternate version of someone in such a divergent timeline is extremely rare,since every possible outcome exists,there could be alternate chains of events that lead to you.Its just the majority of realities with such a big divergence wouldn't have you
- Maybe such time/world travelers are drawn to timelines/realities in which they exist?

I always find it disturbing when the heroes go to another dimension/universe/timeline/whatever and entirely screw it over in order to save their home-reality.

Would it be more noble to not save their home-reality?
- The more interesting question might be: where are the natives of that other reality who fight just as hard to save it from them?
  - Fringe.

What bugs me about alternate universes is when characters return from alternate universes and automatically assume that they are in their original reality. I know that if I had just been in a separate reality from my own I would question for the rest of my life if I was actually home or just in another reality with slightly different characteristics.
- This is explored in Sliders, where the heroes apparently get home but start noticing tiny differences everywhere. At first it looks like they're just incapable of adjusting to normal life again, but then they notice the Golden Gate Bridge is blue...
  - Also in Sliders when they DO get home then decide it isn't because the squeaky fence at the main character's house no longer squeaks.(Then of course we see that it had just been oiled...)

So, for every decision we make, there are different universes that follow each different decision. Considering the fact that me make tons of different decisions a day, even trivial ones like whether we should wear the blue shirt and the red, doesn't that mean that there are possibly millions upon millions of alternate universes out there? And if so, how are people in certain stories find a certain one?

That's part of what makes finding an alternate universe so complicated in the first place.
Personally I think when the decision makes absolutely no difference in the long run the two universes merge back together. Like if no one remembers whether or not you wore the red shirt or the blue shirt and no pictures or video were taken.
As mentioned below, there's a theory that there's millions of alternate universes out there where the only practical difference is that a tree in the Amazon rainforest has a slightly different leaf in each one. Of course, it also depends on the nature of the choice as well; choosing, say, a blue shirt over a red shirt is a slight difference, but if all you're really planning on doing is bumming around the house playing video games all day either way then it's not going to matter what shirt you wear in the long run. As such, like the troper above says, it's technically an alternate universe but in all practical terms it's basically the same one, so if you were to leave Universe!Shirt-Red and appear in Universe!Shirt-Blue and your counterpart in Shirt-Blue was to end up in Shirt-Red, theoretically both of you would more or less slot back into your lives as if you've never left.

Say I'm talking to my alternate self through a wormhole. In theory every possible conversation we could have has an alternate universe associated with it. But there are already two universes involved! Are there simply two universes for every conversation, or a whole multiverse for each?

- I once mused on something like that; that not only are there an infinite amount of universes based on alternate decisions, there are an infinite amount of universes based on their relationship with other universes, just going out into infinity.
- Can time-travel in some wormholes have anything to do with Alternate Universes?

Can alternate universes have alternate universes? Let me use an example to explain better. Hypothetically, I have a chance to ask someone out, but I don't. In an alternate universe, I DO ask. Then, the AU version of the person I asked has the choice to say yes or no. Said person says yes. Is there an alternate universe where the already AU version of the person says no? I would assume so, but I'm quite confused.

- Absolutely; each decision splits off into a new branch, leading to new decisions, which split off to new branches, and so forth. There's actually a theory that suggests that there are an amazing number of universes where the only practical difference between them and the 'home' universe (for our purposes being this one) is that a tree in the Amazon rainforest has a slightly different leaf.
  - That's the Garden of Forking Paths theory.

So in alternate universes is there such a concept as a main timeline? Like would the ultimate results of the first timeline be viewed as the original and most valid universe and without it all others would cease to exist, or are all timelines equally valid and independent of one another?

- All universes are equally valid and independent. It's like a forking path: the original path splits off into two independent and valid paths, both of which stem from the original, but neither of which are a "true" continuation of the original. In a sense, the original path ceases to exist.
- It is all the POV of each Character: Leonardo1 builds a machine and visits Leonardo 2 in the Universe Next Door. Both of them think they are Leonardo1 from Uiverse1.

But what about choices made on logic? If I decide "I could eat this ice cream, but I'm allergic to milk so I shouldn't", is an alternate universe formed where I eat it anyway and inflict pain upon myself? Would there be billions of alternate universes where everyone functions on Insane Troll Logic?

Theoretically you could make any choice and thus have a different universe split off from that choice, but practically some choices are always going to be limited by certain factors. In this case, presumably the 'choice' to eat the ice-cream or not is going to be limited by the fact that you are aware of your allergy to milk, are thus are aware that the ice-cream will cause you pain and discomfort if not outright death, are not a masochist who enjoys the sensation of pain that will be caused by eating the ice-cream, do not wish to risk death by eating the ice-cream, and so on. Theoretically, you could say 'screw the pain, I like the taste so I'll eat it anyway', but practically the 'choice' that is on offer will mostly likely essentially cease to be a choice to all intents and purposes, since commonsense dictates that you're probably not going to eat the ice cream in any case, and thus a different universe won't be split off from it.